Non-volatile memory device having nanocrystal floating gate and method of fabricating same

ABSTRACT

Non-volatile memory devices and methods of fabricating thereof are disclosed herein. An exemplary non-volatile memory device includes a heterostructure disposed over a substrate. A gate structure traverses the heterostructure, such that the gate structure separates a source region and a drain region of the heterostructure and a channel region is defined between the source region and the drain region. The non-volatile memory device further includes a nanocrystal floating gate disposed in the channel region of the heterostructure between a first nanowire and a second nanowire. The first nanowire and the second nanowire extend between the source region and the drain region.

This is a divisional application of U.S. patent application Ser. No.15/220,171, filed Jul. 26, 2016, the entire disclosure of which ishereby incorporated by reference.

BACKGROUND

Non-volatile memory generally refers to any memory or storage that canretain stored data even when no power is applied. Exemplary non-volatilememories include flash memories, which are widely used in memory cardsand USB drives to store data and to transfer data between a computer andother digital devices, such as cameras and mobile phones. Flash memoriesoften implement floating gate transistors, which generally include ametal-oxide-semiconductor field effect transistor (MOSFET) capacitivelycoupled to a number of secondary gates (such as control gates). Becausethe floating gate is electrically isolated from the secondary gates, anycharge trapped on the floating gate is retained for a long period oftime without any power supply. Charge stored on the floating gate can bealtered by applying voltages to source, drain, and/or secondary gates.Nanocrystal charge trap structures are currently being explored forproviding the charge trapping region in the floating gate, as suchstructures can improve charge retention, operate at room temperature,and facilitate quick access. Although existing non-volatile memorydevices implementing nanocrystal charge trap structures and methods forfabricating such have been generally adequate for their intendedpurposes, they have not been entirely satisfactory in all respects

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 is a flow chart of a method for fabricating a non-volatile memorydevice according to various aspects of the present disclosure.

FIGS. 2-14 are fragmentary diagrammatic views of a non-volatile memorydevice 200, in portion or entirety, at various fabrication stages, suchas those associated with the method of FIG. 1, according to variousaspects of the present disclosure.

FIG. 15A and FIG. 15B are fragmentary diagrammatic cross-sectional viewsof another non-volatile memory device, in portion or entirety, which canbe fabricated according to the method of FIG. 1.

FIG. 16A and FIG. 16B are fragmentary diagrammatic cross-sectional viewsof yet another non-volatile memory device, in portion or entirety, whichcan be fabricated according to the method of FIG. 1.

FIG. 17A and FIG. 17B are fragmentary diagrammatic cross-sectional viewsof yet another non-volatile memory device, in portion or entirety, whichcan be fabricated according to the method of FIG. 1.

DETAILED DESCRIPTION

The present disclosure relates generally to non-volatile memory devices,and more particularly, to non-volatile memory devices having nanocrystalfloating gates and methods for fabricating non-volatile memory deviceshaving nanocrystal floating gates.

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact.

In addition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a feature on, connected to, and/or coupled toanother feature in the present disclosure that follows may includeembodiments in which the features are formed in direct contact, and mayalso include embodiments in which additional features may be formedinterposing the features, such that the features may not be in directcontact. In addition, spatially relative terms, for example, “lower,”“upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,”“up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for easeof the present disclosure of one features relationship to anotherfeature. The spatially relative terms are intended to cover differentorientations of the device including the features.

FIG. 1 is a flow chart of a method 100 for fabricating an integratedcircuit device according to various aspects of the present disclosure.In the present embodiment, method 100 fabricates an integrated circuitdevice that includes a non-volatile memory device. At block 105, aheterostructure is formed over a substrate. The heterostructure includesat least one semiconductor layer pair having a first semiconductor layerand a second semiconductor layer disposed over the first semiconductorlayer. The second semiconductor layer is different than the firstsemiconductor layer. At block 110, a gate structure is formed over aportion of the heterostructure, such that the gate structure separates asource region and a drain region of the heterostructure and a channelregion is defined between the source region and the drain region. Thegate structure includes a dummy gate stack, sacrificial gate spacers,and gate spacers. At block 115, epitaxial source/drain features areformed in the source region and the drain region. At block 120, thesacrificial gate spacers are removed from gate structure, therebyforming first opening in gate structure that exposes portion ofheterostructure. At block 125, the second semiconductor layer is removedfrom exposed portions of the heterostructure. At block 130, the dummygate stack is removed from the gate structure, thereby forming a secondopening in the gate structure that exposes a remaining portion of thesecond semiconductor layer. At block 135, an oxide layer is formed overthe remaining portion of the second semiconductor layer. At block 240, agate stack is formed in the second opening of the gate structure. Atblock 145, the method 100 may continue to complete fabrication of thenon-volatile memory device. Additional steps can be provided before,during, and after method 100, and some of the steps described can bemoved, replaced, or eliminated for additional embodiments of method 100.The discussion that follows illustrates various embodiments of anintegrated circuit device having a non-volatile memory device that canbe fabricated according to method 100.

FIGS. 2-14 are fragmentary diagrammatic views of a non-volatile memorydevice 200, in portion or entirety, at various fabrication stages (suchas those associated with method 100) according to various aspects of thepresent disclosure. In particular, FIG. 2 is a perspective,three-dimensional view of non-volatile memory device 200 in an X-Y-Zplane. FIGS. 3-13 are diagrammatic cross-sectional views of non-volatilememory device 200 in an X-Z plane, and FIG. 14 is a diagrammaticcross-sectional view of non-volatile memory device 200 in a Y-Z plane.Examples of non-volatile memory include non-volatile random accessmemory (NVRAM), flash memory, electrically erasable programmable readonly memory (EEPROM), electrically programmable read-only memory(EPROM), and other suitable memory types. In various embodiments,non-volatile memory device 200 is included in a microprocessor, memorycell, and/or other integrated circuit device. In some implementations,non-volatile memory device 200 may be a portion of an integrated circuit(IC) chip, a system on chip (SoC), or portion thereof, that includesvarious passive and active microelectronic devices such as resistors,capacitors, inductors, diodes, metal-oxide semiconductor field effecttransistors (MOSFET), complementary metal-oxide semiconductor (CMOS)transistors, high voltage transistors, high frequency transistors, othersuitable components, or combinations thereof. FIGS. 2-14 have beensimplified for the sake of clarity to better understand the inventiveconcepts of the present disclosure. Additional features can be added innon-volatile memory device 200, and some of the features described belowcan be replaced, modified, or eliminated in other embodiments ofnon-volatile memory device 200.

In FIG. 2 and FIG. 3, non-volatile memory device 200 includes asubstrate (wafer) 202. In the depicted embodiment, substrate 202 is asilicon substrate. Alternatively or additionally, substrate 202 includesgermanium, an alloy semiconductor (for example, SiGe), another suitablesemiconductor material, or combinations thereof. Alternatively,substrate 202 is a semiconductor-on-insulator substrate, such as asilicon-on-insulator (SOI) substrate, a silicon germanium-on-insulator(SGOI) substrate, or a germanium-on-insulator (GOI) substrate.Semiconductor-on-insulator substrates can be fabricated using separationby implantation of oxygen (SIMOX), wafer bonding, and/or other suitablemethods. Substrate 202 can include various doped regions (not shown)depending on design requirements of non-volatile memory device 200. Insome implementations, substrate 202 includes p-type doped regions (forexample, p-type wells) doped with p-type dopants, such as boron (forexample, BF₂), indium, other p-type dopant, or combinations thereof. Insome implementations, substrate 202 includes n-type doped regions (forexample, n-type wells) doped with n-type dopants, such as phosphorus,arsenic, other n-type dopant, or combinations thereof. In someimplementations, substrate 202 includes doped regions formed with acombination of p-type dopants and n-type dopants. The various dopedregions can be formed directly on and/or in substrate 202, for example,providing a p-well structure, an n-well structure, a dual-wellstructure, a raised structure, or combinations thereof. Doping may beimplemented using a process such as ion implantation or diffusion invarious steps and techniques.

Non-volatile memory device 200 further includes a heterostructure 205disposed over substrate 202. In FIG. 2, heterostructure 205 (alsoreferred to as a fin and/or fin structure) extends along an x-direction,having a width defined in a y-direction and a height defined in az-direction. A channel region 206, source region 207, and drain region208 are defined in heterostructure 205, where channel region 206 (alsoreferred to as a gate region herein) is disposed between source region207 and drain region 208 (also generally referred to as source/drainregions). In FIG. 2 and FIG. 3, heterostructure 205 includes asemiconductor layer stack having a length that extends in thex-direction (in other words, horizontally), such that channel region 206extends horizontally between source region 207 and drain region 208. Thesemiconductor layer stack includes various semiconductor layers, such asa semiconductor layer 210, a semiconductor layer 215, a semiconductorlayer 220, and a semiconductor layer 225. Though semiconductor layer 210is depicted as a separate layer, the present disclosure contemplatesembodiments where semiconductor layer 210 is an extension of substrate202 (for example, where portions of substrate 202 are removed to formsemiconductor layer 210 extending from substrate 202). The semiconductorlayers include any suitable material, such as silicon, germanium,silicon germanium, other suitable material, or combinations thereof. Thesemiconductor layers can include same or different materials, etchingrates, constituent atomic percentages, constituent weight percentages,thicknesses (heights), and/or configurations depending on designrequirements of non-volatile memory device 200. In some implementations,the semiconductor layer stack includes at least one semiconductor layerpair having a first semiconductor layer and a second semiconductor layerdisposed over the first semiconductor layer, where the secondsemiconductor layer is different than the first semiconductor layer. Forexample, in the depicted embodiment, heterostructure 205 includes asemiconductor layer pair 205A that includes semiconductor layer 210 andsemiconductor layer pair 215 and a semiconductor layer pair 205B thatincludes semiconductor layer 220 and semiconductor layer 225, wheresemiconductor layer 210 and semiconductor layer 220 include a samematerial and semiconductor layer 215 and semiconductor layer 225 includea same material. The semiconductor layer stack can include more or lesssemiconductor layers and/or configurations depending on designrequirements of non-volatile memory device.

In some implementations, the semiconductor layer stack includessemiconductor layers having different etching rates. For example,semiconductor layer 210 and semiconductor layer 220 include a materialhaving a first etching rate, and semiconductor layer 215 andsemiconductor layer 225 include a material having a second etching rate.The material of semiconductor layer 215 and semiconductor layer 225 mayexhibit a high etch rate relative to the material of semiconductor layer210 and semiconductor layer 220, or vice versa. In some implementations,the semiconductor layer stack includes alternating semiconductor layers,such as semiconductor layers composed of a first material andsemiconductor layers composed of a second material. For example,heterostructure 205 can include a semiconductor layer stack thatalternates silicon layers and silicon germanium layers (for example,Si/SiGe/Si/SiGe from a bottom to a top of heterostructure 205). In suchimplementations, semiconductor layer 210 and semiconductor layer 220 aresilicon layers, and semiconductor layer 215 and semiconductor layer 225are silicon germanium layers. In some implementations, the semiconductorlayer stack includes semiconductor layers of the same material but withalternating constituent atomic percentages, such as semiconductor layershaving a constituent of a first atomic percent and semiconductor layershaving the constituent of a second atomic percent. For example,heterostructure 205 can include a semiconductor layer stack thatincludes silicon germanium layers having alternating silicon atomicpercentages and/or germanium atomic percentages (for example,Si_(a)Ge_(b)/Si_(x)Ge_(y)/Si_(a)Ge_(b)/Si_(x)Ge_(y) from a bottom to atop of heterostructure 205). In such implementations, semiconductorlayer 210 and semiconductor layer 220 are silicon germanium layershaving a first silicon atomic percentage and/or a first germanium atomicpercentage, and semiconductor layer 215 and semiconductor layer 225 aresilicon germanium layers having a second silicon atomic percentageand/or a second germanium atomic percentage that is different than thefirst silicon atomic percentage and/or first germanium atomicpercentage. In some implementations, a germanium atomic percentage ofsemiconductor layer 215 and semiconductor layer 225 ranges from about20% to about 60%, while a germanium atomic percentage of semiconductorlayer 210 and semiconductor layer 220 ranges from about 0% to about 30%.In some implementations, a silicon atomic percentage of semiconductorlayer 215 and semiconductor layer 225 ranges from about 40% to about80%, while a silicon atomic percentage of semiconductor layer 220 andsemiconductor layer 230 ranges from about 70% to about 100%. Forexample, semiconductor layer 210 and semiconductor layer 220 are silicongermanium layers having a silicon atomic percentage of about 80% and agermanium atomic percentage of about 20%, while semiconductor layer 215and semiconductor layer 225 are silicon germanium layers having asilicon atomic percentage of about 50% and a germanium atomic percentageof about 50%.

Heterostructure 205 is formed over substrate 202 using any suitableprocess. In some implementations, semiconductor layer 210 is epitaxiallygrown on substrate 202, semiconductor layer 215 is epitaxially grown onsemiconductor layer 210, semiconductor layer 220 is epitaxially grown onsemiconductor layer 215, and semiconductor layer 225 is epitaxiallygrown on semiconductor layer 220. An epitaxy process can use CVDdeposition techniques (for example, vapor-phase epitaxy (VPE) and/orultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy, other suitableepitaxial growth processes, or combinations thereof. In someimplementations, a lithography and/or etching process is performed toform the fin-like structure of heterostructure 205, such thatheterostructure 205 extends from substrate 202. The lithography processcan include resist coating (for example, spin-on coating), soft baking,mask aligning, exposure, post-exposure baking, developing the resist,rinsing, drying (for example, hard baking), other suitable processes, orcombinations thereof. Alternatively, the lithography process can beimplemented or replaced by other methods, such as maskless lithography,electron-beam writing, ion-beam writing, and/or nanoimprint technology.The etching process can include a dry etching process, wet etchingprocess, other suitable etching process, or combinations thereof. Insome implementations, heterostructure 205 is formed by forming a resistlayer over semiconductor layer 225, exposing the resist to a pattern,and developing the resist to form a masking element including theresist. The masking element is then used to etch (for example, by areactive ion etch (RIE)) semiconductor layer 225, semiconductor layer220, semiconductor layer 215, and semiconductor layer 210 to formheterostructure 205. In another example, heterostructure 205 is formedby a double-patterning lithography (DPL) process. DPL is a method ofconstructing a pattern on a substrate by dividing the pattern into twointerleaved patterns. DPL allows enhanced feature (for example, fin)density. Various DPL methodologies may be used including double exposure(such as using two mask sets), resist freezing, extreme ultraviolet(EUV) lithography, other suitable processes, or combinations thereof.

An isolation feature(s) 230 is formed over and/or in substrate 202 toisolate various regions, such as various device regions, of non-volatilememory device 200. For example, isolation features 230 separate andisolate heterostructure (fin) 205 from other heterostructures (notshown) formed over substrate 202. Isolation features 230 include siliconoxide, silicon nitride, silicon oxynitride, other suitable isolationmaterial, or combinations thereof. Isolation features 230 can includedifferent structures, such as shallow trench isolation (STI) structures,deep trench isolation (DTI) structures, and/or local oxidation ofsilicon (LOCOS) structures. In some implementations, isolation features230 include STI features that define and electrically isolateheterostructure 205 from other active device regions and/or passivedevice regions. For example, STI features can be formed by etching atrench in substrate 202 (for example, by using a dry etch process and/orwet etch process) and filling the trench with insulator material (forexample, by using a chemical vapor deposition process or a spin-on glassprocess). A chemical mechanical polishing (CMP) process may be performedto remove excessive insulator material and/or planarize a top surface ofisolation features 230. In another example, STI features can be formedby depositing an insulator material over substrate 202 after formingheterostructure 205 (in some implementations, such that the insulatormaterial layer fills gaps (trenches) between heterostructures) andetching back the insulator material layer to form isolation features230. In some embodiments, STI features include a multi-layer structurethat fills the trenches, such as a silicon nitride layer disposed over athermal oxide liner layer.

In FIG. 4 and FIG. 5, a gate structure 235 is formed overheterostructure 205. Gate structure 235 includes a dummy gate stack(represented by a dummy gate electrode 240, and in some implementations,a dummy gate dielectric), sacrificial gate spacers 244, and gate spacers246. Gate structure 235 wraps a portion of heterostructure 205 in theY-Z plane, particularly wrapping a portion of channel region 206. In theX-Z plane and the X-Y plane, gate structure 235 interposes source region207 and drain region 208 of heterostructure 205. In the depictedembodiment, dummy gate electrode 240 includes polysilicon or othersuitable dummy gate material. In implementations where the dummy gatestack includes a dummy gate dielectric disposed between dummy gateelectrode 240 and heterostructure 205, the dummy gate dielectricincludes a dielectric material (for example, silicon oxide), high-kdielectric material, other suitable dielectric material, or combinationsthereof. Examples of high-k dielectric material include HfO₂, HfSiO,HfSiON, HfTaO, HfTiO, HfZrO, zirconium oxide, aluminum oxide, hafniumdioxide-alumina (HfO₂—Al₂O₃) alloy, other suitable high-k dielectricmaterials, or combinations thereof. The dummy gate stack may includenumerous other layers, for example, capping layers, interface layers,diffusion layers, barrier layers, hard mask layers, or combinationsthereof. In an example, a capping layer, such as a TiN capping layer, isdisposed between the dummy gate dielectric (or heterostructure 205 wherethe dummy gate dielectric is omitted from gate structure 235) and dummygate electrode 240.

Gate structure 235 is formed by deposition processes, lithographyprocesses, etching processes, other suitable processes, or combinationsthereof. For example, in FIG. 4, a deposition process is performed toform a dummy gate electrode layer over substrate 202, particularlyheterostructure 205 and isolation features 230. In some implementations,a deposition process is performed to form a dummy gate dielectric layerover heterostructure 205 before forming the dummy gate electrode layer,and the dummy gate electrode layer is formed over the dummy gatedielectric layer. The deposition process includes CVD, physical vapordeposition (PVD), atomic layer deposition (ALD), high density plasma CVD(HDPCVD), metal organic CVD (MOCVD), remote plasma CVD (RPCVD), plasmaenhanced CVD (PECVD), low-pressure CVD (LPCVD), atomic layer CVD(ALCVD), atmospheric pressure CVD (APCVD), plating, other suitablemethods, or combinations thereof. A lithography patterning and etchingprocess is then performed to pattern the dummy gate electrode layer(and, in some implementations, the dummy gate dielectric layer) to formthe dummy gate stack including dummy gate electrode 240 of gatestructure 235, such that dummy gate electrode 240 wraps a portion ofchannel region 206. The lithography patterning processes include resistcoating (for example, spin-on coating), soft baking, mask aligning,exposure, post-exposure baking, developing the resist, rinsing, drying(for example, hard baking), other suitable processes, or combinationsthereof. Alternatively, the lithography exposing process is assisted,implemented, or replaced by other methods, such as maskless lithography,electron-beam writing, or ion-beam writing. In yet another alternative,the lithography patterning process implements nanoimprint technology.The etching processes include dry etching processes, wet etchingprocesses, other etching methods, or combinations thereof.

In FIG. 5, sacrificial gate spacers 244 and gate spacers 246 are formedadjacent to the dummy gate stack (for example, dummy gate electrode 240)of gate structure 235. Sacrificial gate spacers 244 are disposedadjacent to (for example, along sidewalls of) dummy gate electrode 240,and gate spacers 246 are disposed adjacent to (for example, alongsidewalls of) sacrificial gate spacers 244. Sacrificial gate spacers 244and gate spacers 246 each include a dielectric material, such as siliconoxide, silicon nitride, silicon oxynitride, silicon carbide, othersuitable material, or combinations thereof. Sacrificial gate spacers 244include a dielectric material different than gate spacers 246. Forexample, sacrificial gate spacers 244 include silicon oxide (SiO₂), andgate spacers 246 include silicon nitride (for example, Si₃N₄). Inanother example, sacrificial gate spacers 244 include silicon nitride,and gate spacers 246 include silicon carbon nitride (SiCN). In someimplementations, sacrificial gate spacers 244 include a dielectricmaterial having a different etching rate than gate spacers 246. Forexample, sacrificial gate spacers 244 include a first dielectricmaterial having a first etching rate (such as silicon oxide), and gatespacers 246 include a second dielectric material having a second etchingrate (such as silicon nitride). In some implementations, sacrificialgate spacers 244 include a material having a high etch rate relative toa material of gate spacers 246. In some implementations, sacrificialgate spacers 244 and/or gate spacers 246 include a multi-layerstructure, such as a multi-layer structure including a silicon nitridelayer and a silicon oxide layer. Sacrificial gate spacers 244 and gatespacers 246 are formed by any suitable process. For example, in thedepicted embodiment, a silicon oxide layer can be deposited overheterostructure 205 and subsequently anisotropically etched (forexample, dry etched) to form sacrificial gate spacers 244, and a siliconnitride layer can be deposited over heterostructure 205 and subsequentlyetched (for example, dry etched) to form gate spacers 246.

In FIG. 6, epitaxial source features and epitaxial drain features(referred to as epitaxial source/drain features) are formed insource/drain regions of heterostructure 205. For example, asemiconductor material is epitaxially (epi) grown on exposed portions ofheterostructure 205, forming an epitaxial source/drain feature 248 insource region 207 and an epitaxial source/drain feature in drain region208. An epitaxy process can use CVD deposition techniques (for example,vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD)),molecular beam epitaxy, other suitable epitaxial growth processes, orcombinations thereof. The epitaxy process can use gaseous and/or liquidprecursors, which interact with the composition of heterostructure 205(in other words, interact with semiconductor layer 210, semiconductorlayer 215, semiconductor layer 220, and semiconductor layer 225).Epitaxial source/drain features 248 may be doped with n-type dopantsand/or p-type dopants. In some implementations, where non-volatilememory device 200 is configured as an n-type device (for example, havingan n-channel), epitaxial source/drain features 248 are silicon epitaxiallayers or silicon carbon epitaxial layers, where the silicon epitaxiallayers or silicon carbon epitaxial layers are doped with phosphorous,other n-type dopant, or combinations thereof (for example, forming aSi:P epitaxial layer or a Si:C:P epitaxial layer). In someimplementations, where non-volatile memory device 200 is configured as ap-type device (for example, having a p-channel), epitaxial source/drainfeatures 248 are silicon germanium (SiGe) layers, where the SiGe layersare doped with boron, other p-type dopant, or combinations thereof (forexample, forming a Si:Ge:B epitaxial layer). In some implementations,epitaxial source/drain features 248 include materials and/or dopantsthat achieve desired tensile stress and/or compressive stress in channelregion 206. In some implementations, epitaxial source/drain features 248are doped during deposition by adding impurities to a source material ofthe epitaxy process. In some implementations, epitaxial source/drainfeatures 248 are doped by an ion implementation process subsequent to adeposition growth process. In some implementations, annealing processesare performed to activate dopants in epitaxial source/drain features 248and/or other source/drain regions of non-volatile memory device 200 (forexample, heavily doped source/drain regions and/or lightly dopedsource/drain (LDD) regions).

An inter-level dielectric (ILD) layer 250 can be formed over substrate202, for example, by a deposition process (such as chemical vapordeposition (CVD), physical vapor deposition (PVD), atomic layerdeposition (ALD), high density plasma CVD (HDPCVD), metal organic CVD(MOCVD), remote plasma CVD (RPCVD), plasma enhanced CVD (PECVD),low-pressure CVD (LPCVD), atomic layer CVD (ALCVD), atmospheric pressureCVD (APCVD), plating, other suitable methods, or combinations thereof).ILD layer 250 includes a dielectric material, such as silicon oxide,silicon nitride, silicon oxynitride, TEOS formed oxide, phosphosilicateglass (PSG), borophosphosilicate glass (BPSG), low-k dielectricmaterial, other suitable dielectric material, or combinations thereof.Exemplary low-k dielectric materials include fluorinated silica glass(FSG), carbon doped silicon oxide, Black Diamond® (Applied Materials ofSanta Clara, Calif.), Xerogel, Aerogel, amorphous fluorinated carbon,Parylene, BCB (bis-benzocyclobutenes), SiLK (Dow Chemical, Midland,Mich.), polyimide, other proper materials, or combinations thereof. Inthe depicted embodiment, ILD layer 250 is a low-k dielectric layer. Insome implementations, ILD layer 250 can include a multilayer structurehaving multiple dielectric materials. Subsequent to the deposition ofILD layer 250, a chemical mechanical polishing (CMP) process may beperformed, such that a top portion of gate structure 235 is reached(exposed), particularly, a top portion of gate electrode 280.

In FIGS. 7-14, a gate replacement process is performed to replace adummy gate (here, dummy gate electrode 240) of gate structure 235 with agate, such as a metal gate stack. During the gate replacement process,heterostructure 205 can be processed using horizontal gate-all-around(GAA) processing technologies, fin-like field effect transistor (FinFET)technologies, and/or channel-on-oxide transistor technologies tofabricate at least one nanocrystal floating gate (also referred to as acharge storing nanocrystal) disposed above and/or between at least onenanowire channel. In some implementations, heterostructure 205 isprocessed to fabricate a nanowire disposed between nanocrystal floatinggates, improving threshold voltage control (programmability) of thenanowire. Further, by fabricating nanocrystal floating gates during thegate replacement process, nanocrystal floating gates automaticallyself-align between a source and a drain (such as source region 207 anddrain region 208), and further automatically self-align between achannel (such as a nanowire channel) and a control gate. Accordingly,fabricating nanocrystal floating gates is easily and cost-effectivelyintegrated into existing fabrication processes. Different embodimentsmay have different advantages, and no particular advantage isnecessarily required of any embodiment.

In FIG. 7, a portion of gate structure 235 is removed to expose portionsof heterostructure 205. In some implementations, an etching process (forexample, a selective wet etching process) removes sacrificial gatespacers 244 to form openings 252 within gate structure 235 that exposeportions of heterostructure 205 in channel region 206, such as portionsof semiconductor layer 210, semiconductor layer 215, semiconductor layer220, and semiconductor layer 225 disposed between dummy gate electrode240 and gate spacers 246. The etching process is a dry etching process,a wet etching process, or combinations thereof. In some implementations,due to a high etch rate of the material of sacrificial gate spacers 244relative to the material of gate spacers 246, the etching process canremove sacrificial gate spacers 244 while eliminating or minimizing anyremoval of gate spacers 246. Various etching parameters can be tuned toselectively etch sacrificial gate spacers 244, such as etchantcomposition, etching temperature, etching solution concentration,etching time, etching pressure, source power, radio frequency (RF) biasvoltage, RF bias power, etchant flow rate, other suitable etchingparameters, or combinations thereof. In some implementations,sacrificial gate spacers 244 may be etched using etch reactants having ahigh etch selectivity with respect to the material of sacrificial gatespacers 244. For example, in implementations where sacrificial gatespacers 244 include silicon oxide, a wet etching process using ahydrofluoric acid solution can selectively etch sacrificial gate spacers244 without etching gate spacers 246. In another example, inimplementations where sacrificial gate spacers 244 include siliconnitride, a wet etching process using a phosphoric acid solution canselectively etch sacrificial gate spacers 244 without etching gatespacers 246.

In FIG. 8, portions of heterostructure 205 are selectively removed toform nanowires. For example, exposed semiconductor layer 215 andsemiconductor layer 225 are selectively etched within channel region206, such that a nanowire 210A and a nanowire 220A are formed in channelregion 206. The etching process is a dry etching process, a wet etchingprocess, or combinations thereof. In some implementations, whereheterostructure 205 alternates between first semiconductor layers andsecond semiconductor layers having different materials, an etchingprocess selectively removes the second material layers. For example, insome implementations, due to a high etch rate of the material ofsemiconductor layer 215 and semiconductor layer 225 relative to thematerial of semiconductor layer 210 and semiconductor layer 220, theetching process can remove exposed portions of semiconductor layer 215and semiconductor layer 225 while eliminating or minimizing any removalof exposed portions of semiconductor layer 210 and semiconductor layer220. Various etching parameters can be tuned to selectively etchsemiconductor layer 215 and semiconductor layer 225, such as etchantcomposition, etching temperature, etching solution concentration,etching time, etching pressure, source power, RF bias voltage, RF biaspower, etchant flow rate, other suitable etching parameters, orcombinations thereof. In some implementations, semiconductor layer 215and semiconductor layer 225 may be etched using etch reactants having ahigh etch selectivity with respect to the material of semiconductorlayer 210 and semiconductor layer 220. For example, in someimplementations, a dry etching process (such as a reactive ion etching(RIE) process) utilizing a fluorine-containing gas (for example, SF₆)can selectively etch semiconductor layer 215 and semiconductor layer225. In some implementations, a ratio of the fluorine-containing gas toan oxygen-containing gas (for example, O₂), an etching temperature (forexample, above or below about 80° C.), and/or an RF power may be tunedto selectively etch silicon germanium or silicon. In another example, insome implementations, a wet etching process utilizing an etchingsolution that includes NH₄OH, H₂O₂ and H₂O can selectively etchsemiconductor layer 215 and semiconductor layer 225.

In FIG. 8A, dummy gate electrode 240 and gate spacers 246 are partiallytransparent, revealing that nanowire 210A and nanowire 220A extendhorizontally between source region 207 and drain region 208, such thatnanowire 210A and nanowire 220A constitute channels of non-volatilememory device 200. As further depicted in FIG. 8A, remaining portions ofsemiconductor layer 215 and semiconductor layer 225, which underliedummy gate electrode 240, form a semiconductor island 215A (alsoreferred to as nanocrystal 215A) disposed over nanowire 210A and asemiconductor island 225A (also referred to as nanocrystal 225A)disposed over nanowire 220A. In implementations where semiconductorlayer 210 and semiconductor layer 220 are silicon layers andsemiconductor layer 215 and semiconductor layer 225 are silicongermanium layers, nanowire 210A and nanowire 220A are silicon channels,and semiconductor island 215A and semiconductor island 225A are silicongermanium nanocrystals disposed over silicon channels. The etchingprocess may also remove portions of semiconductor layer 215 andsemiconductor layer 225 underlying dummy gate electrode 240, such that awidth of semiconductor island 215A and semiconductor island 225A is lessthan a width of dummy gate electrode 240 as depicted in FIG. 8A. Thoughnanowire 210A, nanowire 220A, semiconductor island 215A, andsemiconductor island 225A have rectangular shapes, the presentdisclosure contemplates nanowire 210A, nanowire 220A, semiconductorisland 215A, and semiconductor island 225A having any configuration,shape, and/or size (including cylindrical, triangular, hexagonal,trapezoidal, or other desired shapes) depending on design requirementsof non-volatile memory device 200.

In FIG. 9, the dummy gate stack of gate structure 235 (here, dummy gateelectrode 240) is removed, thereby forming a trench (opening) 255.Opening 255 exposes a portion of channel region 206, and in particular,exposes nanowire 210A, semiconductor island 215A, nanowire 220A, andsemiconductor island 225A in channel region 206. The dummy gate stack isremoved using any suitable process. In some implementations, an etchingprocess selectively removes dummy gate electrode 240 (and, in someimplementations, the dummy gate dielectric). The etching process is adry etching process, a wet etching process, or combinations thereof. Aselective etching process (such as a selective wet etch and/or aselective dry etch) can be tuned as described herein, such that dummygate electrode 240 has an adequate etch rate relative to gate spacers246 and other device features in opening 255 (for example, nanowire210A, semiconductor island 215A, nanowire 220A, and semiconductor island225A).

In FIG. 10, an oxide layer 270 is formed over exposed portions ofchannel region 206. For example, oxide layer 270 is formed over nanowire210A, semiconductor island 215A, nanowire 220A, and semiconductor island225A. In some implementations, oxide layer 270 is a semiconductor oxidelayer, such as a silicon oxide layer. Portions of nanowire 210A,semiconductor island 215A, nanowire 220A, and semiconductor island 225Acan be oxidized to form oxide layer 270. For example, in someimplementations, a thermal oxidation process is performed on nanowire210A, semiconductor island 215A, nanowire 220A, and semiconductor island225A. An oxidation rate of semiconductor island 215A and semiconductorisland 225A (in some implementations, silicon germanium islands) ishigher than an oxidation rate of nanowire 210A and nanowire 220A (insome implementations, silicon nanowires), such that greater portions ofsemiconductor island 215A and semiconductor island 225A are consumedduring the thermal oxidation process. Accordingly, a thickness of oxidelayer 270 on semiconductor island 215A and semiconductor island 225A isgreater than a thickness of oxide layer 270 on nanowire 210A andnanowire 220A. The thermal oxidation process is conducted in an oxygenambient, a steam ambient, other suitable ambient, or combinationsthereof. In some implementations, the thermal oxidation process is a wetoxidation process. In some implementations, the thermal oxidationprocess is tuned to achieve higher oxidation rates of semiconductorlayer 215 and semiconductor layer 225, for example, by performing thethermal oxidation process at a temperature less than about 900° C.

In FIG. 11, portions of oxide layer 270 are removed, forming nanocrystalfloating gates of non-volatile memory device 200. For example, oxidelayer 270 is removed from nanowire 210A and nanowire 220A using anetching process, such that a nanocrystal floating gate 260A (which is aremaining portion of semiconductor island 215 after various processing,such as the described etching and oxidation processes) is surrounded bya tunnel oxide layer 272A and a nanocrystal floating gate 260B (which isa remaining portion of semiconductor island 225 after variousprocessing, such as the described etching and oxidation processes) issurrounded by a tunnel oxide layer 272B. The etching process is a dryetching process, a wet etching process, or combination thereof. In someimplementations, a wet etching process is tuned (for example, bycontrolling etching time) to ensure that oxide layer 270 is sufficientlyremoved from nanowire 210A and nanowire 220A. Nanocrystal floating gate260A serves as a lower floating gate and nanocrystal floating gate 260Bserves as an upper floating gate. In the depicted embodiment,nanocrystal floating gate 260A and nanocrystal floating gate 260B havecylindrical shapes, though the present disclosure contemplatesnanocrystal floating gate 260A and nanocrystal floating gate 260B havingany configuration, shape, and/or size (including rectangular,triangular, hexagonal, trapezoidal, or other desired shapes) dependingon design requirements of non-volatile memory device 200. As depicted inFIG. 11, tunnel oxide layer 272A surrounds nanocrystal floating gate260A, and tunnel oxide layer 272B surrounds nanocrystal floating gate260B, and nanocrystal floating gate 260A and nanocrystal floating gate260B are vertically self-aligned over nanowire 210A and nanowire 220A.

In FIG. 12 and FIG. 13, a metal gate stack of gate structure 235 isformed in opening (trench) 255. FIG. 14 depicts non-volatile memorydevice 200 at the same fabrication stage as FIG. 13 (after forming themetal gate stack of gate structure 235), with non-volatile memory device200 viewed in the Y-Z plane (perpendicular to a channel direction, whichextends in the x-direction). The metal gate stack includes a gatedielectric 275 and a gate electrode 280. The metal gate stackconstitutes a control gate of non-volatile memory device 200. The metalgate stack of gate structure 235 may include numerous other layers, forexample, capping layers, interface layers, diffusion layers, barrierlayers, hard mask layers, or combinations thereof. In FIG. 12, gatedielectric 275 is formed over exposed portions of non-volatile memorydevice 200 in opening 255. For example, gate dielectric 275 conforms toexposed surfaces of opening 255, including exposed surfaces of nanowire210A and nanowire 220A, exposed surfaces of gate spacers 246, andexposed surfaces of tunnel oxide layer 272A and tunnel oxide layer 272B.In FIG. 13, gate electrode 280 is formed over gate dielectric 275. Gatedielectric 275 and gate electrode 280 are formed by various depositionprocesses. In some implementations, an atomic layer deposition (ALD)process deposits a gate dielectric layer over exposed surfaces inopening 255, and an ALD process deposits a gate electrode layer over thegate dielectric layer. The gate dielectric layer and the gate electrodelayer may conform to the exposed surfaces in opening 255, providing gatedielectric 275 and gate electrode 280 as depicted. A CMP process can beperformed to remove excess gate electrode layer, planarizing the metalgate stack.

Gate dielectric 275 includes a dielectric material, such as siliconoxide, high-k dielectric material, other suitable dielectric material,or combinations thereof. Examples of high-k dielectric material includeHfO₂, HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, zirconium oxide, aluminumoxide, hafnium dioxide-alumina (HfO₂—Al₂O₃) alloy, other suitable high-kdielectric materials, or combinations thereof. In some implementations,gate dielectric 275 is a high-k dielectric layer. In someimplementations, gate dielectric 275 includes an interfacial layer (suchas a silicon oxide layer), and a high-k dielectric layer disposed overthe interfacial layer. Gate electrode 280 includes a conductivematerial, such as polysilicon, Al, Cu, Ti, Ta, W, Mo, TaN, NiSi, CoSi,TiN, WN, TiAl, TiAlN, TaCN, TaC, TaSiN, other conductive material, orcombinations thereof. In some implementations, gate electrode 280includes a work function layer, which is a conductive layer tuned tohave a desired work function (such as an n-type work function or ap-type work function), and a conductive layer formed over the workfunction layer. In various examples, the work function layer includesTa, TaN, NiSi, CoSi, TiN, WN, TiAl, TiAlN, TaCN, TaC, TaSiN, othermaterial suitable for achieving a desired work function, or combinationsthereof. The conductive layer formed over the work function layerincludes a suitable conductive material, such as aluminum, tungsten, orcopper. The conductive layer may additionally or collectively includepolysilicon, titanium, tantulum, metal alloys, other suitable materials,or combinations thereof. In some implementations, silicide features areformed in gate regions and/or source/drain regions. For example,silicide features can be formed over epitaxial on source/drain features248 and additionally on gate electrode 280 when gate electrode 280includes polysilicon. The silicide features are formed by a silicidationprocess, such as a self-aligned silicide (salicide) process.

Various contacts are formed to facilitate operation of non-volatilememory device 200. In FIG. 13 and FIG. 14, an ILD layer 282, similar toILD layer 250, can be formed over substrate 202 (in the depictedembodiment, over ILD layer 250 and gate structure 235). Contacts can beformed in ILD layer 250 and/or ILD layer 282. For example, a contact 284is electrically coupled with a control gate of non-volatile memorydevice 200 (particularly gate electrode 280), a contact 286 iselectrically coupled to source region 207 (particularly epitaxialsource/drain feature 248), and a contact 288 is electrically coupled todrain region 208 (particularly epitaxial source/drain feature 248).Contacts 284, 286, and 288 include a conductive material, such as metal.Metals include aluminum, aluminum alloy (such as aluminum/silicon/copperalloy), copper, copper alloy, titanium, titanium nitride, tantalum,tantalum nitride, tungsten, polysilicon, metal silicide, other suitablemetals, or combinations thereof. The metal silicide may include nickelsilicide, cobalt silicide, tungsten silicide, tantalum silicide,titanium silicide, platinum silicide, erbium silicide, palladiumsilicide, or combinations thereof. In some implementations, ILD layer250, ILD layer 282, contact 284, contact 286, and/or contact 288 are aportion of a multilayer interconnect (MLI) feature disposed oversubstrate 202. The MLI feature electrically couples various componentsof non-volatile memory device 200, such that the various components areoperable to function as specified by design requirements of non-volatilememory device 200. In some implementations, the MLI includes at leastone bit line, at least one word line, at least one source line, and/orat least one erase line. In some implementations, contact 284 iselectrically coupled with a word line, contact 286 is electricallycoupled with a source line, and contact 288 is electrically coupled witha bit line. Contact 284, contact 286, and contact 288 may be considereda portion of the respective word line, source line, and bit line. TheMLI feature can include a combination of metal layers and ILD layersconfigured to form vertical interconnect features, such as contactsand/or vias, and/or horizontal interconnect features, such as lines. Thevarious conductive features include materials similar to contacts 284,286, and 288. In some implementations, a damascene process and/or dualdamascene process is used to form a copper-based multilayerinterconnection structure. Nonvolatile memory device 200 can undergosubsequent processing to complete fabrication.

In FIG. 13 and FIG. 14, non-volatile memory device 200 is configuredwith a horizontal gate-all-around transistor, which includeshorizontally stacked nanowires having nanocrystal floating gatesdisposed between and/or above the nanowires. For example, nanowire 220Ais disposed between nanocrystal floating gate 260A and nanocrystalfloating gate 260B, where nanocrystal floating gate 260A is disposedover nanowire 210A and nanocrystal floating gate 260B is disposed overnanowire 210B. Gate structure 235 (in particular, the metal gate stackincluding gate dielectric 275 and gate electrode 280) substantiallysurrounds nanocrystal floating gate 260A and nanocrystal floating gate260B. However, nanocrystal floating gate 260A and nanocrystal floatinggate 260B are isolated from gate electrode 280 (a control electrode) bygate dielectric 275 and respective tunnel oxide layer 272A and tunneloxide layer 272B, such that nanocrystal floating gate 260A andnanocrystal floating gate 260B can store electrical charge (electrons orholes). Nanocrystal floating gate 260A and nanocrystal floating gate260B are thus also referred to as charge storing (or storage)nanocrystals. By implementing nanocrystal floating gates above and belownanowire 220A (here, nanocrystal floating gate 260B and nanocrystalfloating gate 260A, respectively), a programmable threshold voltage innanowire 220A is better controlled during operation, particularly whencompared to non-volatile memory devices that implement a singlenanocrystal floating gate above (or below) a nanowire. By fabricatingnanocrystal floating gate 260A and nanocrystal floating gate 260B duringthe gate replacement process, nanocrystal floating gate 260A andnanocrystal floating gate 260B are self-aligned between source region207 and drain region 208, and are further self-aligned between the metalgate stack (in particular, the control gate provided by gate dielectric275 and gate electrode 280) and nanowire 220A (which provides a nanowirechannel). Furthermore, In some implementations (such as depicted in FIG.13 and FIG. 14), by fabricating nanocrystal floating gate 260A andnanocrystal floating gate 260B during the gate replacement process,nanocrystal floating gate 260A and nanocrystal floating gate 260B aregenerally cigar-shaped. In such implementations, nanocrystal floatinggate 260A and nanocrystal floating gate 260B have a length (in thex-direction) that is greater than a height (in the z-direction) and awidth (in the y-direction). In some implementations, nanocrystalfloating gate 260A and nanocrystal floating gate 260B have a generallyoval-shaped cross-section in the X-Z plane and a generally circle-shapedcross-section in the Y-Z plane.

In operation, biasing heterostructure 205 (specifically source region207 and drain region 208) and gate structure 235 (specifically gateelectrode 280) via contacts 284, 286, and/or 288 causes nanocrystalfloating gate 260A and/or nanocrystal floating gate 260B tocharge/discharge electrons (or holes), such that one bit of data may bewritten to, read from, or erased from nanocrystal floating gate 260A andnanocrystal floating gate 260B. In some implementations, non-volatilememory device 200 is configured with additional nanocrystal floatinggates, such that non-voltatile memory device 200 can store more than onebit of information. When a bit is written to, a negative charge(electron) is placed on nanocrystal floating gate 260A and/ornanocrystal floating gate 260B. When a bit is erased, charge is removedfrom the bit, returning nanocrystal floating gate 260A and/ornanocrystal floating gate 260B to an uncharged state.Charging/discharging nanocrystal floating gate 260A and nanocrystalfloating gate 260B modifies a threshold voltage of nanowire 210A and/ornanowire 220A, such that non-volatile memory device 200 can achieve twodifferent logic states. In some implementations, nanocrystal floatinggate 260A and nanocrystal floating gate 260B yield a lower thresholdvoltage when charged and a higher threshold voltage when uncharged. Insome implementations, non-volatile memory device 200 operates as a flashmemory.

During write operations and erase operations, non-volatile memory device200 can utilize Fowler-Nordheim (“FN”) tunneling of electrons (or holes)from nanowire 210A and/or nanowire 220A through tunnel oxide layer 272Aand tunnel oxide layer 272B to nanocrystal floating gate 260A andnanocrystal floating gate 260B. For example, a write operation isachieved by grounding source region 207 and drain region 208 viarespective contact 286 and contact 288 and applying a positive voltage(for example, 5 volts) to gate electrode 280 via contact 284. A positivepotential on gate electrode 280 effects FN tunneling of electrons fromnanowire 210A and/or nanowire 220A through tunnel oxide layer 272Aand/or tunnel oxide layer 272B to nanocrystal floating gate 260A and/ornanocrystal floating gate 260B. The negative charge on nanocrystalfloating gate 260A and nanocrystal floating gate 260B modifies (forexample, increases) a threshold voltage of nanowire 210A and/or nanowire220A, such that a logical zero (0) can be read from non-volatile memorydevice 200. In another example, an erase operation is achieved byapplying a positive voltage (for example, 5 volts) to source region 207and drain region 208 via respective contact 286 and contact 288 andgrounding gate electrode 280 via contact 284. A positive potential onsource region 207 and drain region 208 effects FN tunneling of electronsfrom nanocrystal floating gate 260A and/or nanocrystal floating gate260B through tunnel oxide layer 272A and/or tunnel oxide layer 272B tonanowire 210A and/or nanowire 220A. The lack of charge on nanocrystalfloating gate 260A and nanocrystal floating gate 260B modifies(decreases) a threshold voltage of nanowire 210A and/or nanowire 220A,such that a logical one (1) can be read from non-volatile memory device200.

FIG. 15A and FIG. 15B are fragmentary diagrammatic cross-sectional viewsof an integrated circuit device that includes a non-volatile memorydevice 300, in portion or entirety, according to various aspects of thepresent disclosure. FIG. 15A depicts non-volatile memory device 300 inan X-Z plane, and FIG. 15B depicts non-volatile memory device 300 in aY-Z plane. Non-volatile memory device 300 is similar in many respects tonon-volatile memory device 200. Accordingly, similar features in FIG.15A and FIG. 15B and FIGS. 2-14 are identified by the same referencenumerals for clarity and simplicity. In FIG. 15A and FIG. 15B,non-volatile memory device 300 includes four nanowires and fournanocrystal floating gates, when compared to non-volatile memory device200. For example, non-volatile memory device 300 further includes ananowire 210B, a nanowire 220B, a nanocrystal floating gate 260C(surrounded by a tunnel oxide layer 272C), and a nanocrystal floatinggate 220D (surrounded by a tunnel oxide layer 272D). Non-volatile memorydevice 300 can be fabricated using horizontal gate-all-around (GAA)processing technologies similar to non-volatile memory device 200. Forexample, non-volatile memory device 300 can begin with heterostructure205 including an additional set of semiconductor layers (in particular,another semiconductor layer 210, another semiconductor layer 215,another semiconductor layer 220, and another semiconductor layer 225)disposed over semiconductor layer 225 and proceed through thefabrication processes as described with reference to FIGS. 2-14, suchthat nanowire 210B is fabricated from the another semiconductor layer210, nanocrystal floating gate 260C is fabricated from the anothersemiconductor layer 215, nanowire 220B is fabricated from the anothersemiconductor layer 220, and nanocrystal floating gate 260D isfabricated from the another semiconductor layer 225. FIG. 15A and FIG.15B have been simplified for the sake of clarity to better understandthe inventive concepts of the present disclosure. Additional featurescan be added in non-volatile memory device 300, and some of the featuresdescribed below can be replaced, modified, or eliminated in otherembodiments of non-volatile memory device 300.

FIG. 16A and FIG. 16B are fragmentary diagrammatic cross-sectional viewsof an integrated circuit device that includes a non-volatile memorydevice 400, in portion or entirety, according to various aspects of thepresent disclosure. FIG. 16A depicts non-volatile memory device 400 inan X-Z plane, and FIG. 16B depicts non-volatile memory device 400 in aY-Z plane. Non-volatile memory device 400 is similar in many respects tonon-volatile memory device 200. Accordingly, similar features in FIG.16A and FIG. 16B and FIGS. 2-14 are identified by the same referencenumerals for clarity and simplicity. In FIG. 16A and FIG. 16B,non-volatile memory device 400 can be processed using fin-like fieldeffect transistor (FinFET) processing technologies to achievenon-volatile memory device 400 having a FinFET transistor that includesa fin channel having a nanocrystal floating gate disposed thereover. Forexample, non-volatile memory device 400 includes nanowire 210Aconfigured as a fin extending from substrate 202 and nanocrystalfloating gate 260A disposed thereover. In such implementations,non-volatile memory device 400 can begin with heterostructure 205including only semiconductor layer 210 and semiconductor layer 215 andproceed through the fabrication processes as described with reference toFIGS. 2-14 to achieve non-volatile memory device 400 as depicted. Insome implementations, semiconductor layer 210 has an initial thicknessgreater than an initial thickness of semiconductor layer 215. Assemiconductor layer 210 is fabricated to form a fin, the fin may beconfigured as nanowire 210A of non-volatile memory device 400. In someimplementations, the fin is configured as a tall nanowire, for example,having a length (in the x-direction) that is much larger than a height(in the z-direction). FIG. 16A and FIG. 16B have been simplified for thesake of clarity to better understand the inventive concepts of thepresent disclosure. Additional features can be added in non-volatilememory device 400, and some of the features described below can bereplaced, modified, or eliminated in other embodiments of non-volatilememory device 400.

FIG. 17A and FIG. 17B are fragmentary diagrammatic cross-sectional viewsof an integrated circuit device that includes a non-volatile memorydevice 500, in portion or entirety, according to various aspects of thepresent disclosure. FIG. 17A depicts non-volatile memory device 500 inan X-Z plane, and FIG. 17B depicts non-volatile memory device 500 in aY-Z plane. Non-volatile memory device 500 is similar in many respects tonon-volatile memory device 200. Accordingly, similar features in FIG.17A and FIG. 17B and FIGS. 2-14 are identified by the same referencenumerals for clarity and simplicity. In FIG. 17A and FIG. 17B,non-volatile memory device 500 can be processed using channel-on-oxideprocessing technologies to achieve non-volatile memory device 500 havinga channel-on-oxide transistor that includes a channel disposed betweennanocrystal floating gates. For example, non-volatile memory device 500includes nanowire 220A configured as a channel layer disposed betweennanocrystal floating gate 260A and nanocrystal floating gate 260B, andmore particularly, disposed between tunnel oxide layer 272A and tunneloxide layer 272B. In such implementations, non-volatile memory device500 can begin with heterostructure 205 including semiconductor layer 210having a thickness T1 and semiconductor layer 220 having a thickness T2(where T2 is greater than T1) and proceed through the fabricationprocesses as described with reference to FIGS. 2-14 to fabricate acharge storing nanocrystal above and below a channel (provided bysemiconductor layer 220) of non-volatile memory device 500. Assemiconductor layer 220 is fabricated to form the channel, the channelmay be configured as nanowire 220A of non-volatile memory device 500. Insome implementations, the channel is configured as a tall nanowire, forexample, having a length (in the x-direction) that is much larger than aheight (in the z-direction). FIG. 17A and FIG. 17B have been simplifiedfor the sake of clarity to better understand the inventive concepts ofthe present disclosure. Additional features can be added in non-volatilememory device 500, and some of the features described below can bereplaced, modified, or eliminated in other embodiments of non-volatilememory device 500.

The present disclosure provides for many different embodiments. Anexemplary method for fabricating a non-volatile memory device includesforming a heterostructure over a substrate. The heterostructure includesat least one semiconductor layer pair having a first semiconductor layerand a second semiconductor layer disposed over the first semiconductorlayer, the second semiconductor layer being different than the firstsemiconductor layer. A gate structure having a dummy gate is formed overa portion of the heterostructure, such that the gate structure separatesa source region and a drain region of the heterostructure and a channelregion is defined between the source region and the drain region. Duringa gate replacement process, a nanocrystal floating gate is formed in thechannel region from the second semiconductor layer. In someimplementations, during the gate replacement process, a nanowire is alsoformed in the channel region from the first semiconductor layer.

In some implementations, the gate structure further includes gatespacers and sacrificial gate spacers. The method can further includeremoving the sacrificial gate spacers to form a first opening in thegate structure that exposes a portion of the heterostructure; andremoving the second semiconductor layer from the exposed portion of theheterostructure in the first opening. In some implementations, thesecond semiconductor layer is removed by a selective etching process,where the second semiconductor layer has a higher etch rate relative tothe first semiconductor layer. In some implementations, the dummy gateis removed to form a second opening in the gate structure that exposes aremaining portion of the second semiconductor layer in the channelregion. An oxide layer can be formed over the remaining portion of thesecond semiconductor layer. In some implementations, forming the oxidelayer includes oxidizing a portion of the remaining portion of thesecond semiconductor layer and a portion of the first semiconductorlayer exposed by the second opening; and removing the oxidized portionof the first semiconductor layer.

In some implementations, the heterostructure includes a firstsemiconductor layer pair and a second semiconductor layer pair disposedover the first semiconductor layer pair, the first semiconductor layerpair and the second semiconductor layer pair each having the firstsemiconductor layer and the second semiconductor layer. During the gatereplacement process, a first nanocrystal floating gate can be formed ofthe second semiconductor layer in the first semiconductor layer pair, asecond nanocrystal floating gate can be formed of the secondsemiconductor layer in the second semiconductor layer pair, and ananowire can be formed of the first semiconductor layer of the secondsemiconductor layer pair. The nanowire may be disposed between the firstnanocrystal floating gate and the second nanocrystal floating gate.

The gate replacement process can include forming a gate dielectric overthe first semiconductor layer and the nanocrystal floating gate in thechannel region, and forming a gate electrode over the gate dielectric,wherein the gate includes the gate dielectric and the gate electrode. Insome implementations, the method further includes forming a control gatecontact electrically coupled to the control gate, a source contactelectrically coupled to the source, and a drain contact electricallycoupled to the drain. In some implementations, the method furtherincludes forming epitaxial source/drain features in the source regionand the drain region of the heterostructure before the gate replacementprocess.

Another exemplary method for fabricating a non-volatile memory deviceincludes forming a semiconductor layer stack over a substrate. Thesemiconductor layer stack includes at least one first semiconductorlayer of a first semiconductor material and at least one secondsemiconductor layer of a second semiconductor material, the secondsemiconductor material being different than the first semiconductormaterial. The method further includes forming a gate structure over achannel region of the semiconductor layer stack. The gate structureincludes a dummy gate stack, sacrificial gate spacers, and gate spacers.The sacrificial gate spacers are removed to form a first opening in thegate structure that exposes a first portion of the semiconductor layerstack in the channel region. The at least one second semiconductor layercan be removed from the exposed first portion of the semiconductor layerstack. The dummy gate stack is removed to form a second opening in thegate structure that exposes a second portion of the semiconductor layerstack in the channel region, wherein the exposed second portion includesat least one second semiconductor layer island. The method can furtherinclude forming an oxide layer over the at least one secondsemiconductor layer island. A metal gate stack may be formed in thesecond opening of the gate structure.

In some implementations, the first opening is defined between the dummygate stack and the gate spacers, and the second opening is definedbetween the gate spacers. Forming the gate structure can include formingthe sacrificial gate spacers adjacent to the dummy gate, wherein thesacrificial gate spacers include a first spacer material, and formingthe gate spacers adjacent to the sacrificial gate spacers. The gatespacers include a second spacer material having a different etching ratethan the first spacer material. Forming the metal gate stack in thesecond opening of the gate structure can include forming a gatedielectric over the first semiconductor layer in the exposed secondportion of the semiconductor layer stack and the oxide layer, andforming a gate electrode over the gate dielectric.

In some implementations, the oxide layer is formed by oxidizing portionsof the at least one second semiconductor layer island, such that the atleast one second semiconductor layer island is surrounded by the oxidelayer. Forming the oxide layer can further includes oxidizing portionsof the at least one first semiconductor layer in the exposed secondportion of the semiconductor layer stack, and removing the oxidizedportions of the at least one first semiconductor layer. In someimplementations, the gate structure traverses the semiconductor layerstack in a manner that separates a source region and a drain region ofthe semiconductor layer stack, the channel region extending horizontallybetween the source region and the drain region. The method can furtherinclude forming epitaxial source/drain features over the semiconductorlayer stack in the source region and the drain region. In someimplementations, the semiconductor layer stack includes alternatingfirst semiconductor layers and second semiconductor layers, such thatafter forming the oxide layer, the channel region includes a nanowire ofthe first semiconductor material disposed between nanocrystal floatinggates of the second semiconductor material.

An exemplary non-volatile memory device includes a heterostructuredisposed over a substrate. A gate structure is disposed over a portionof the heterostructure, the gate structure traversing theheterostructure, such that the gate structure separates a source regionand a drain region of the heterostructure and a channel region isdefined between the source region and the drain region. A nanocrystalfloating gate is disposed in the channel region of the heterostructurebetween a first nanowire and a second nanowire, wherein the firstnanowire and the second nanowire extend between the source region andthe drain region. In some implementations, the heterostructure includesat least one first semiconductor layer of a first semiconductor materialand at least one second semiconductor layer of a second semiconductormaterial, the second semiconductor material being different than thefirst semiconductor material. The first nanowire and the second nanowiremay be of the first semiconductor material, while the nanocrystalfloating gate may be of the second material. In some implementations,the second nanowire is disposed between the nanocrystal floating gateand another nanocrystal floating gate.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A non-volatile memory device comprising: aheterostructure disposed over a substrate; a gate structure traversingthe heterostructure, such that the gate structure separates a sourceregion and a drain region of the heterostructure, wherein a channelregion is defined between the source region and the drain region; and ananocrystal floating gate disposed in the channel region of theheterostructure between a first nanowire and a second nanowire, whereinthe first nanowire and the second nanowire extend between the sourceregion and the drain region.
 2. The non-volatile memory device of claim1, wherein: the heterostructure includes at least one firstsemiconductor layer of a first semiconductor material and at least onesecond semiconductor layer of a second semiconductor material, thesecond semiconductor material being different than the firstsemiconductor material; the first nanowire and the second nanowire areof the first semiconductor material; and the nanocrystal floating gateis of the second semiconductor material.
 3. The non-volatile memorydevice of claim 2, wherein the first semiconductor material includessilicon and the second semiconductor material includes silicon andgermanium.
 4. The non-volatile memory device of claim 1, furthercomprising epitaxial source/drain features disposed on the source regionand the drain region of the heterostructure.
 5. The non-volatile memorydevice of claim 1, wherein the nanocrystal floating gate is surroundedby a tunnel oxide layer.
 6. The non-volatile memory device of claim 1,wherein the nanocrystal floating gate has a first cross-sectional shapealong a length of the nanocrystal floating gate and a secondcross-sectional shape along a width of the nanocrystal floating gate,wherein the first cross-sectional shape is different than the secondcross-sectional shape.
 7. The non-volatile memory device of claim 6,wherein the first cross-sectional shape is oval-shaped and the secondcross-sectional shape is circle-shaped.
 8. The non-volatile memorydevice of claim 1, wherein the first nanowire extends from the substrateand the second nanowire is suspended over the first nanowire.
 9. Thenon-volatile memory device of claim 1, wherein the first nanowire andthe second nanowire are suspended over the substrate.
 10. Thenon-volatile memory device of claim 1, wherein the first nanowire has afirst thickness, the second nanowire has a second thickness, and thefirst thickness is different than the second thickness.
 11. Anon-volatile memory device comprising: a transistor that includes ananowire disposed between a first nanocrystal floating gate and a secondnanocrystal floating gate; wherein the nanowire includes a firstsemiconductor material and the first nanocrystal floating gate and thesecond nanocrystal floating gate include a second semiconductor materialthat is different than the first semiconductor material; wherein acontrol gate of the transistor surrounds the nanowire, the firstnanocrystal floating gate, and the second nanocrystal floating gate;wherein the control gate traverses a source and a drain of thetransistor, the nanowire extending between the source and the drain,such that a channel of the transistor is defined in the nanowire; andwherein the source and the drain each include at least one first layerincluding the first semiconductor material and at least one second layerincluding the second semiconductor material.
 12. The non-volatile memorydevice of claim 11, wherein the first nanocrystal floating gate and thesecond nanocrystal floating gate are vertically aligned between thesource and the drain.
 13. The non-volatile memory device of claim 11,wherein the first nanocrystal floating gate and the second nanocrystalfloating gate are surrounded by an oxide material.
 14. The non-volatilememory device of claim 11, wherein the first semiconductor materialincludes silicon and the second semiconductor material includes siliconand germanium.
 15. The non-volatile memory device of claim 11, whereinthe nanowire extends continuously from one of the at least one firstlayer of the source to one of the at least one first layer of the drain.16. The non-volatile memory device of claim 15, wherein the firstnanocrystal floating gate is horizontally aligned with a first one ofthe at least one second layer of the source and a first one of the atleast one second layer of the drain, and the second nanocrystal floatinggate is horizontally aligned with a second one of the at least onesecond layer of the source and a second one of the at least one secondlayer of the drain.
 17. The non-volatile memory device of claim 11,wherein the control gate includes a gate dielectric and a gateelectrode.
 18. A non-volatile memory device comprising: a transistorthat includes a nanowire disposed between a first nanocrystal floatinggate and a second nanocrystal floating gate; wherein the nanowireincludes a first semiconductor material and the first nanocrystalfloating gate and the second nanocrystal floating gate include a secondsemiconductor material, wherein the first semiconductor material and thesecond semiconductor material include different constituent atomicpercentages of the same material; wherein a control gate of thetransistor surrounds the nanowire, the first nanocrystal floating gate,and the second nanocrystal floating gate; wherein the control gatetraverses a source and a drain of the transistor, the nanowire extendingbetween the source and the drain, such that a channel of the transistoris defined in the nanowire; and wherein the source and the drain eachinclude at least one first layer including the first semiconductormaterial and at least one second layer including the secondsemiconductor material.
 19. A non-volatile memory device comprising: afirst nanowire; a first nanocrystal floating gate disposed over thefirst nanowire; a second nanowire disposed over the first nanocrystalfloating gate; a second nanocrystal floating gate disposed over thesecond nanowire; and a control gate disposed over the first nanowire,the first nanocrystal floating gate, the second nanowire, and the secondnanocrystal floating gate, wherein the control gate interposes afin-like source feature and a fin-like drain feature.
 20. Thenon-volatile memory device of claim 19, wherein the first nanocrystalfloating gate and the second nanocrystal floating gate are verticallyself-aligned between the fin-like source feature and the fin-like drainfeature.